Conventional encoding methods use normally modulation formats such that the number of available code words, i.e. the number of modulation constellation points, is a power of two. In this case an integer number of bits can be encoded per symbol. Recently, further modulation formats have been proposed where the number of available code words is not a power of two and the information content per symbol comprises a non-integer number of bits. Examples of these modulation formats are for instance DPSK-3ASK or M-QAM. wherein M is an any square number. These modulation formats with M available code words have an information capacity of ld(M) bits per symbol, wherein ld(X) is the base-2 logarithm of X. In case that the number M of available code words is not of a power of two using conventional encoding methods or encoders only n=[ld(M)]bits can be encoded in each symbol, wherein [X] is the largest integer number smaller than or equal to X. Accordingly with this conventional encoding methods out of the available M code words only 2n code words are utilized and M−2n possible code words remain unused.
Accordingly, it is an object of the present invention to provide an apparatus and a method increasing the transmission capacity when using encoding methods with modulation formats where the number of available code words is not a power of two.